Sintered alloy valve guide and its production method

ABSTRACT

To provide a sintered alloy valve guide having high thermal conductivity and excellent wear resistance, which can be used in engines subjected to a large thermal load due to downsizing, direct injection and supercharging, the sintered alloy has a composition comprising by mass 10-90% of Cu, 0-10% of Cr, 0-6% of Mo, 0-8% of V, 0-8% of W, and 0.5-3% of C, the balance being substantially Fe and inevitable impurities, the total amount of Cr, Mo, V and W being 2% or more, and a structure comprising an Fe-based alloy phase, a Cu or Cu-based alloy phase, and a graphite phase.

FIELD OF THE INVENTION

The present invention relates to a valve guide for guiding the opening and closing of an engine valve, and its production method, particularly to a high-thermal-conductivity valve guide capable of suppressing valve temperature elevation, and its production method.

BACKGROUND OF THE INVENTION

Though valve guides for automobile engines were conventionally castings, they have been replaced by sintered alloy parts produced in near-net shapes by powder metallurgy without needing machining, and their production has been increasing at high ratios. As an example of the sintered alloy valve guides, JP 6-306554 discloses a sintered alloy valve guide having a pearlite-based matrix having a composition comprising by weight 1-4% of C, 1.5-6% of Cu, and 0.1-0.8% of P, the balance being Fe and inevitable impurities, in which Fe—C—P compounds and free graphite are dispersed.

In recent gasoline engines for automobiles, attempts have been carried out to improve combustion efficiency by combining various technologies of downsizing, direct injection, supercharging, etc., for lower fuel consumption, lower emission, and higher power. Improvement in the combustion efficiency is achieved by reducing various losses, and attention is paid particularly to exhaust loss, which occupies a high ratio of the loss. To reduce the exhaust loss, compression ratios have been increased. Higher compression ratios are inevitably accompanied by abnormal combustion such as knocking, etc. because of higher engine temperatures, needing the cooling of combustion chambers. Particularly near exhaust valves exposed to high temperatures, cooling must be improved, requiring valve guides acting to cool valves to have high cooling capability.

Valve guides having high valve-cooling capability are made of, for example, brass, but they suffer poor material properties such as insufficient wear resistance, etc., and cost disadvantages such as higher machining cost than conventional iron-based valve guides, etc. Accordingly, sintered alloy valve guides having high valve-cooling capability and wear resistance while meeting cost requirements are desired.

As an iron-based sintered alloy valve guide having better wear resistance than a conventional level for recent engines having higher performance and higher fuel efficiency, JP 11-323512 A discloses a sintered iron-based alloy valve guide produced by mixing, molding and sintering Fe powder, C powder and Cu—Ni alloy powder, which has a structure in which fine precipitates of a free graphite phase having an average particle size of 30 μm or less are dispersed in a matrix of an Fe-based alloy phase bound by a Cu-based alloy phase, the Fe-based alloy phase having a composition comprising by weight 20-40% of Cu, 0.6-14% of Ni, and 1.0-3.0% of C, the balance being Fe and inevitable impurities.

OBJECT OF THE INVENTION

In view of the above problems, an object of the present invention is to provide a sintered alloy valve guide having high thermal conductivity and excellent wear resistance, which can be used in engines subjected to a large thermal load due to downsizing, direct injection and supercharging, and a method for producing such a sintered alloy valve guide.

DISCLOSURE OF THE INVENTION

As a result of intensive research on sintered alloy valve guides for engines, the inventor has found that the use of an Fe-based alloy powder containing elements selected from Cr, Mo, W and V, which is coated with Cu, provides a sintered alloy valve guide having high wear resistance and thermal conductivity.

Thus, the sintered alloy valve guide of the present invention has a composition comprising by mass 10-90% of Cu, 0-10% of Cr, 0-6% of Mo, 0-8% of V, 0-8% of W, and 0.5-3% of C, the balance being substantially Fe and inevitable impurities, the total amount of Cr, Mo, V and W being 2% or more, and a structure comprising an Fe-based alloy phase, a Cu or Cu-based alloy phase, and a graphite phase.

The Fe-based alloy phase is preferably an Fe—Mo—C alloy, an Fe—Cr—Mo—V—C alloy, an Fe—Cr—V—W—C alloy, or an Fe—Cr—Mo—V—W—C alloy. It preferably has a composition comprising by mass 0-10% of Cr, 0-6% of Mo, 0-8% of V, 0-8% of W, and 0.5-1% of C, the balance being substantially Fe and inevitable impurities, the total amount of Cr, Mo, V and W being 2% or more.

The Cu or Cu-based alloy phase is preferably continuous in the structure. The Cu or Cu-based alloy phase preferably has thermal conductivity of 200 W/mK or more.

The sintered alloy valve guide of the present invention is produced by coating prealloy powder having a composition comprising by mass 0-10% of Cr, 0-6% of Mo, 0-8% of V, 0-8% of W, and 0.5-1% of C, the balance being substantially Fe and inevitable impurities, the total amount of Cr, Mo, V and W being 2% or more, with Cu, mixing the Cu-coated prealloy powder with C powder, and then molding and sintering.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical photomicrograph showing the structure of the sintered alloy valve guide of Example 1.

FIG. 2 is a schematic view showing a wear test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The sintered alloy valve guide of the present invention has a composition comprising by mass 10-90% of copper (Cu), 0-10% of chromium (Cr), 0-6% of molybdenum (Mo), 0-8% of vanadium (V), 0-8% of tungsten (W), and 0.5-3% of carbon (C), the balance being iron (Fe) and inevitable impurities, the total amount of Cr, Mo, V and W being 2% or more. The sintered alloy valve guide of the present invention also has a structure comprising an Fe-based alloy phase, a Cu or Cu-based alloy phase, and a graphite phase contributing to wear resistance, thermal conductivity, and self-lubrication. The Fe-based alloy phase comprises Fe as a main component, and the Cu-based alloy phase comprises Cu as a main component.

In the sintered alloy valve guide of the present invention, Cu is an indispensable alloy component to have high thermal conductivity. The thermal conductivity of the sintered alloy valve guide is preferably 30 W/(m·K) or more, more preferably 50 W/mK or more. Less than 10% by mass of Cu generates an insufficient liquid phase, as well as an insufficient Cu or Cu-based alloy phase, failing to obtain a dense sintered alloy having desired thermal conductivity. On the other hand, more than 90% by mass of Cu forms too small an amount of the Fe-based alloy phase, resulting in poor wear resistance. Thus, Cu is 10-90% by mass. Cu is preferably 30% or more by mass and 80% or less by mass, more preferably 75% or less by mass. Because thermal conductivity is mostly governed by the motion of free electrons in crystal grains in the metal, less solid solution elements provide higher thermal conductivity. Accordingly, it is important to reduce the amounts of elements dissolved in Cu. In this sense, Mo, V, W, C, Cr and Fe do not have adverse influence on the thermal conductivity of Cu, because Mo, V, W and C are not substantially dissolved in Cu, and because Cr and Fe form a mixed structure with Cu by cooling from a high temperature at which they are slightly dissolved in Cu. Accordingly, the Cu-based alloy phase in the present invention may be a Cu—Cr alloy, a Cu—Fe alloy or a Cu—Cr—Fe alloy. Each of these Cu-based alloy phases can have thermal conductivity of 200 W/mK or more. Ni is not contained in the present invention, because Ni forms a solid solution with Cu at any ratio, undesirably extremely reducing the thermal conductivity.

Cr, Mo, V and W are dissolved in the Fe-based alloy phase, contributing to improvement in strength and hardness. They further form carbides to improve wear resistance. When the total amount of Cr, Mo, V and W is less than 2.0% by mass, good heat resistance and wear resistance cannot be obtained. On the other hand, when Cr exceeds 10% by mass, when Mo exceeds 6% by mass, or when each of V and W exceeds 8% by mass, excessive or coarse precipitates are formed, weakening the Fe-based alloy phase, increasing the attacking of a mating member, or suffering breakage when pressed into a cylinder head. Accordingly, the amounts of Cr, Mo, V and W added are by mass 0-10% of Cr, 0-6% of Mo, 0-8% of V, and 0-8% of W, the total amount of Cr, Mo, V and W being 2% or more. The upper limit of the total amount of Cr, Mo, V and W is 32% by mass, but it is preferably 16% or less by mass taking into consideration the attacking of a mating member.

C is dissolved in the Fe-based alloy phase or forms carbides, to improve the strength and hardness of the alloy. It is also dispersed as graphite, imparting self-lubrication to the alloy. When C is less than 0.5% by mass, sufficient carbides are not precipitated, failing to obtain the above effects. On the other hand, when C exceeds 3% by mass, excessive carbides or too coarse carbides are precipitated, resulting in reduced toughness, and thus lower performance. Accordingly, C is 0.5-3% by mass.

In the production method of the sintered alloy valve guide of the present invention, Fe-based alloy powder having a composition comprising by mass 0-10% of Cr, 0-6% of Mo, 0-8% of V, 0-8% of W, and 0.5-1% of C, the balance being substantially Fe and inevitable impurities, the total amount of Cr, Mo, V and W being 2% or more, is used as a starting material powder. A Cu component may be added by mixing the Fe-based alloy powder with a Cu or Cu-based alloy powder, or by coating the Fe-based alloy powder with Cu. A Cu coating may be formed by plating the Fe-based alloy powder with Cu, the mechanical alloying of the Fe-based alloy powder with Cu powder, etc., and Cu plating is preferable. The Fe-based alloy powder is preferably formed by water atomization, and subjected to immersion plating in an electroless plating solution to form a predetermined Cu component layer. The C powder is preferably graphite powder having an average particle size of 1-20 μm. The starting material powder may contain stearate, etc. as a parting agent.

The above starting material powders are mixed, and the resultant mixed powder is charged into a die, compression-molded by pressing, etc., degreased, if necessary, and then sintered at 900-1050° C. in vacuum. The sintering temperature of lower than 900° C. fails to obtain a sintered body having a desired structure because of an insufficient liquid phase formed from Cu or its alloy, and the sintering temperature of higher than 1050° C. cannot keep a predetermined shape of the sintered body because of too much a liquid phase formed from Cu or its alloy. The sintering temperature is 900-1050° C.

EXAMPLE 1

Fe-based prealloy powder having a composition comprising by mass 1.33% of Cr, 2.67% of Mo, 4.00% of V and 0.57% of C was electroless-plated with Cu, to form Cu-coated powder (Cu: 45.5% by mass per 100% by mass of the entire powder), and mixed with graphite powder to form mixed powder (C: about 2% per 100% of the entire mixture). 0.5% by mass of zinc stearate was added to and blended with 100% by mass of the mixed powder to form a starting material powder mixture. This starting material powder mixture was charged into a die, and compression-molded by pressing at pressure of 6.5 t/cm² to form a green body, which was degreased, and then sintered at 1000° C. in vacuum to produce a cylindrical sintered body of 15 mm in diameter and 50 mm in height.

FIG. 1 is an optical photomicrograph showing the structure of the sintered body of Example 1. The sintered body had a relatively dense structure comprising relatively coarse Fe-based alloy phase particles 1, a Cu (or Cu-based alloy) phase 2, and relatively fine graphite phase particles 3, with pores 4 slightly observed. It is characterized by a continuous Cu (or Cu-based alloy) phase 2.

[1] Wear Test

A valve guide test piece of 10 mm×50 mm×10 mm was machined from the cylindrical sintered body, and a valve test piece (sliding mating member) of 8 mm in diameter and 30 mm in length, whose one end had an 8-mm-R cylindrical surface, was cut out of an SUH alloy valve material. As shown in FIG. 2, the valve test piece 6 was pushed to the reciprocally moving valve guide test piece 5 under a constant load to evaluate wear resistance. The test conditions were as follows.

Pushing load: 50 N,

Test temperature: 200° C.,

Lubrication: No (dry),

Stroke: 25 mm

Sliding speed: 166 mm/second, and

Test time: 3 hours.

The receding amounts of contact surfaces of the valve guide test piece and the valve test piece after the test were regarded as wear. As a result, the wear in Example 1 was 2.0 μm in the valve guide test piece, and 21.5 μm in the valve test piece.

[2] Measurement of Thermal Conductivity

A disc-shaped test piece of 5.0 mm in diameter and 1.0 mm in thickness was cut out of the cylindrical sintered body, mirror-polished on bottom surfaces, and its thermal conductivity was measured by a laser flash method. The thermal conductivity in Example 1 was 50 W/mK.

Examples 2-8, and Comparative Examples 1-5

Sintered bodies were produced in the same manner as in Example 1, except for changing the Fe-based prealloy composition, the amount of electroless Cu plating, and the amount of C powder added as shown in the column of “Chemical Components” in Table 1. A valve guide test piece for the wear test and a disc-shaped test piece for the thermal conductivity measurement were formed from each sintered body, and subjected to the same wear test and thermal conductivity measurement as in Example 1. The results are shown in Table 1 together with those of Example 1.

TABLE 1 Wear (μm) Thermal Chemical Components (% by mass) Valve Conductivity No. Cr Mo V W C Cu Fe Guide Valve (W/mK) Example 1 0.7 1.4 2.1 — 2.3 45.5 Bal. 2.0 21.5 50 Example 2 0.5 1.1 0.8 1.7 2.1 65.6 Bal. 6.0 12.5 84 Example 3 0.6 0.8 0.5 1.2 0.9 85.2 Bal. 11.0 12.0 123 Example 4 3.6 4.5 1.8 5.4 2.0 10 Bal. 1.8 22.8 32 Example 5 0.7 1.5 2.2 0.6 3.0 46.7 Bal. 2.2 18.5 45 Example 6 9.6 0.8 1.6 — 2.5 20.0 Bal. 3.1 21.2 38 Example 7 — 5.0 — — 0.5 30.5 Bal. 8.5 14.7 62 Example 8 2.0 — 6.2 5.6 1.6 50.2 Bal. 3.2 24.3 55 Com. Ex. 1 0.4 0.2 0.1 0.05 0.4 4.8 Bal. 290.5 0.5 28 Com. Ex. 2 0.2 0.05 — — 0.3 95.2 Bal. 495.6 1.0 355 Com. Ex. 3 12.0 6.8 8.3 9.8 0.2 8.0 Bal. 1.1 86.5 18 Com. Ex. 4 3.4 0.1 5.2 12.0 3.5 10 Bal. 1.3 100.3 16 Com. Ex. 5 17.8 0.4 0.1 0.3 2.0 3.2 Bal. 5.3 59.1 17

It is clear from Examples that when the total amount of alloy elements of Cr, Mo, V and W is 2% or more by mass, the valve guide undergoes less wear. On the other hand, when the total amount of the alloy elements exceeds 16% by mass, the valve suffers drastically increased wear. The thermal conductivity gets higher by increasing the amount of Cu, but it is also influenced by the alloy phase composition, low in Examples 4 and 6 containing large amounts of alloy elements. Because Example 7 contains a small amount of C, the valve guide has small self-lubrication by graphite, suffering more wear, though it has higher thermal conductivity than that of Example 5 containing more Cu. The reason therefor appears to be that C contained in a continuous Cu structure has little influence on decreasing thermal conductivity. In Comparative Examples 1 and 2, in which the total amount of alloy elements is less than 2% by mass, the valves (sliding mates) suffer little wear, but the valve guides per se suffer increased wear. Particularly in Comparative Example 2 containing a small total amount of the alloy elements and more than 90% by mass of the Cu component, the valve guide per se suffers drastically increased wear because of insufficient strength and hardness. Because in Comparative Examples 3-5, any of the alloy elements (all of Cr, Mo, V and W in Comparative Example 3, W in Comparative Example 4, and Cr in Comparative Example 5) is more than the required amount, the valves (mating members) suffer large wear, though the valve guides undergo little wear. Further, they have as insufficiently low thermal conductivity as 20 W/mK or less.

EFFECTS OF THE INVENTION

Because the sintered alloy valve guide of the present invention comprises a wear-resistant Fe-based alloy phase, a Cu or Cu-based alloy phase having excellent thermal conductivity, and a graphite phase having excellent self-lubrication, it has excellent wear resistance and high valve-cooling capability, thereby avoiding abnormal combustion such as knocking, etc. in high-performance, high-load engines, and contributing to improving engine performance. 

1. A sintered alloy valve guide having a composition comprising by mass 10-90% of Cu, 0-10% of Cr, 0-6% of Mo, 0-8% of V, 0-8% of W, and 0.5-3% of C, the balance being substantially Fe and inevitable impurities, the total amount of Cr, Mo, V and W being 2% or more, and having a structure comprising an Fe-based alloy phase, a Cu or Cu-based alloy phase, and a graphite phase.
 2. The sintered alloy valve guide according to claim 1, wherein said Fe-based alloy phase is an Fe—Mo—C alloy, an Fe—Cr—Mo—V—C alloy, an Fe—Cr—V—W—C alloy, or an Fe—Cr—Mo—V—W—C alloy.
 3. The sintered alloy valve guide according to claim 2, wherein said Fe-based alloy phase has a composition comprising by mass 0-10% of Cr, 0-6% of Mo, 0-8% of V, 0-8% of W, and 0.5-1% of C, the balance being substantially Fe and inevitable impurities, the total amount of Cr, Mo, V and W being 2% or more.
 4. The sintered alloy valve guide according to claim 1, wherein said Cu or Cu-based alloy phase is continuous in the structure.
 5. The sintered alloy valve guide according to claim 4, wherein said Cu or Cu-based alloy phase has thermal conductivity of 200 W/mK or more.
 6. A method for producing a sintered alloy valve guide comprising the steps of coating prealloy powder having a composition comprising by mass 0-10% of Cr, 0-6% of Mo, 0-8% of V, 0-8% of W, and 0.5-1% of C, the balance being substantially Fe and inevitable impurities, the total amount of Cr, Mo, V and W being 2% or more, with Cu, mixing the Cu-coated prealloy powder with C powder, and then molding and sintering. 